1. Field of the Invention
The present invention relates to an inkjet image forming apparatus. More particularly, the present invention relates to a method and apparatus for compensating for image quality degradation caused by defective nozzles in an inkjet image forming apparatus.
2. Description of the Related Art
An inkjet image forming apparatus forms images by ejecting ink from an inkjet head onto a printing medium. The inkjet head is placed a predetermined distance apart from the printing medium and moves reciprocally in a direction perpendicular to the transferring direction of the printing medium. Such an inkjet image forming apparatus is referred to as a shuttle type inkjet image forming apparatus. An inkjet head of the shuttle type inkjet image forming apparatus includes a nozzle unit on which a plurality of nozzles that eject ink are formed.
Recently, an inkjet head having a nozzle unit with a length corresponding to the width of a printing medium has been used for high-speed printing. An image forming apparatus operating in this way is referred to as a page width inkjet image forming apparatus. An inkjet head of the page width inkjet image forming apparatus is fixed and only a printing medium is transferred. Accordingly, a driving device of the inkjet image forming apparatus is simple and high-speed printing is possible.
FIG. 1 shows printing patterns when a defective nozzle exists in a conventional inkjet image forming apparatus, and FIGS. 2A through 2D are pixel images for explaining a method of compensating for the defective nozzle in the conventional inkjet image forming apparatus.
Referring to FIG. 1, the inkjet image forming apparatus forms an image by spraying ink I from nozzles 82 formed on a nozzle unit 80 onto a printing medium. When a nozzle 84 is defective, a visible unprinted line is generated on the printing medium as shown in FIG. 1. Such an unprinted line is easily visible, and thus affects printing quality.
A method of compensating for deterioration of image quality due to a missing or not working nozzle is disclosed in U.S. Pat. No. 5,581,284. FIGS. 2A through 2D are the same drawings illustrated in FIGS. 3 through 6 of U.S. Pat. No. 5,581,284. Referring to FIGS. 2A through 2D, the defective nozzle indicates a case when a nozzle does not normally eject ink or is missing in the pixel image as the printing medium is moved in the direction of an arrow. When a defective nozzle ejecting mono (that is, black) ink is detected, ink droplets of other colors (for example, cyan, magenta, and yellow) are sequentially ejected to a pixel region 63 to which the defective nozzle should have ejected black ink. That is, the black color can be represented by printing the cyan, magenta, and yellow ink droplets on the same position of the printing medium. The represented black is called process black (“P” in FIG. 2D) or composite black. However, this method is useful to compensate for a defective nozzle ejecting black ink, but does not compensate for detective nozzles ejecting other colors. Moreover, since the nozzles for cyan, magenta, and yellow ink do not operate when only the black color is printed, the process black cannot be formed using other color nozzles. However, when a color image is printed, the nozzles for cyan, magenta, and yellow ink operate, and the compensation cannot be performed. Further, when one of the nozzles used for compensation malfunctions, other colors such as red (yellow+magenta), green (cyan+yellow), or blue (cyan+magenta) color are printed, and thus printing quality is deteriorated. Accordingly, there is a need for an improve way to compensate for a defective nozzle to improve image quality.